As the level of agricultural mechanization is improved in China, tractors applicable to hilly uplands have been developed to some degree. Hillside tractors not only can improve the degree of mechanization in hilly upland regions, but also can improve the living conditions of workers in mountainous regions. To adapt to the topographical characteristics of hilly uplands, body attitude leveling of hillside tractors is an important requirement for hillside tractors during use. The body leveling of a conventional hillside tractor is controlled manually by an operator; in addition, at present, existing hillside tractors in China are mainly walking tractors and mini-type caterpillar tractors, and there is no relating references or patent search reports refer to a drive axle with a body auto-leveling function for a four-wheel drive tractor.
Up to now, in the patent search carried out with keywords “hillside tractor” and “auto-leveling”, the following four patent publications have been found:    1. Hillside Tractor with Transverse Leveling Body (application No.: CN201520084082.2, publication No.: CN204432812U) introduces a tractor capable of transverse leveling adjustment. In overall layout, the split-type frame is articulated and consists of a front frame, a middle frame and a rear frame, which are articulated together, and a roll-waist turning method is used; a frame leveling detection signal is generated as a mercury switch mounted on the frame is inclined, and the signal is used to control solenoid directional valves, and thereby the relative distance from the reducer of two driving wheels at one side to the frame is adjusted, and leveling control in the transverse direction is realized.    2. Remotely Controlled Hillside Tractor (application No.: CN201010215718.4) introduces a caterpillar tractor that can be remotely controlled to operate on a hilly upland, wherein, the power is transferred via a transmission case directly to the driving wheels of caterpillar tracks, and two hydraulic cylinders on the two sides of the frame are connected to the supporting frames of the left and right track rollers and the tractor body; a body auto-leveling signal is issued from the processor in a remote controller to control the hydraulic oil cylinders to ascend and descend, and thereby transverse leveling of the tractor body is realized.    3. Transmission Mechanism for Hillside Tractor (application No.: CN200920093628.5), in which the power transfer between a transmission semi-axle and a driving wheel is implemented via a sprocket structure, the wheel spacing is adjustable, but there is no tractor body leveling function.    4. Balance Device for Hillside Walking Tractor (application No.: CN201220194143.7) provides a balance control device for a walking tractor, which utilizes left and right hydraulic oil cylinders to adjust the positions of the transmission case and the driving wheels to attain a purpose of body attitude leveling.
It is seen from the result of the patent search: in the patent publication 1, the device can realize transverse vehicle body leveling, but the vehicle frame consists of three pieces articulated together, causing increased traveling instability on a hillside land; in addition, in the transverse leveling process of the drive axle, the distances from the centers of the wheels to the vehicle body are adjusted by turning the wheels and the transmission mechanism around the vehicle body longitudinally; as a result, though the vehicle body is leveled, the axle spacing between front and rear wheels and the wheel spacing between left and right wheels are changed, and consequently the load ratios of the wheels are changed, and finally the wheel adhesion is changed accordingly; besides, since the axle spacing and wheel spacing are changed, the instability of the entire vehicle is increased, and the steering performance parameters of the entire vehicle are also changed due to the change of the wheel spacing and axle spacing parameters, bringing increased difficulties in steering operation by the driver. In the patent publication 2, the positions of the left and right caterpillar tracks with respect to the vehicle body are controlled wirelessly and automatic electrical body leveling control is realized, but the advantage of high power transmission efficiency of a wheeled tractor is missing; in the patent publication 3, the power of the driving wheels of the hillside tractor is transferred by means of a sprocket structure, but the power transmission efficiency of sprocket transmission is not as high as that of gear transmission; in addition, there is no vehicle body auto-leveling function; the patent publication 4 discloses a balance device for a walking tractor, in which the elevations of the driving wheels with respect to the vehicle body are changed by rotating the driving wheels and the reduction gear mechanism around the vehicle body; since the driving wheels and the reduction gear mechanism rotate around the vehicle body in the longitudinal direction, the positions of the driving wheels with respect to the vehicle body in the longitudinal direction are also changed as the elevations of the driving wheels are changed; consequently, for a four-wheel tractor, the vehicle body load on the left and right driving wheels are changed, and the wheel adhesion is also changed; besides, owing to the change of the wheel spacing, the steering performance parameters of the entire vehicle changes, it is adverse to the steering operations; moreover, automatic longitudinal leveling control is not realized.